/Engine/Source/Editor/UnrealEd/Public/Commandlets/AnalyzeReferencedContentStat.h
Assuming DXT1 lightmaps... 4 bits/pixel * width * height = Highest MIP Level * 1.333 MIP Factor for approx usage for a full mip chain Either 1 or 3 textures if we're doing simple or directional (3-axis) lightmap Most lightmaps require a second UV channel which is probably an extra 4 bytes (2 floats compressed to SHORT)
static int32 CalculateLightmapLightingBytesUsed &40; int32 Width, int32 Height, int32 NumVertices, int32 UVChannelIndex &41;